In many applications it is advantageous to attach a back plate to a circuit board. The back plate is fairly rigid, fabricated out of a metal, and has about the same length and width as the circuit board. The circuit board is thin, on the order of 2 to 100 millimeters, and therefore difficult to handle and to work with because it bends and tends to warp. The additional thickness of an assembly of the circuit board attached to the back plate prevents the circuit board from bending and warping. This makes it easier to install components onto the circuit board, and easier to install the assembly into a device. The circuit board is made out of one or more layers. Some of the layers are made out of plastic or out of a plastic and glass compound. These layers have a conductor, such as copper, deposited on both sides to form a particular pattern of tracks that is the printed wiring. Other layers are made of an epoxy material and hold the circuit board together. Components attach to the circuit board and connect to the printed wiring. The back plate also acts as a heat sink to dissipate some of the heat generated by the components.
Typically, the back plate is attached to the circuit board with an epoxy. Some circuit boards can be attached to the back plates with solder. However, this is very expensive and would drastically increase the cost of the assembly. Other circuit boards, for example circuit boards with low glass transition temperatures, cannot be attached to the back plates with solder because this solder connection will melt when this assembly is heated to attach components to the assembly. Once the back plate and circuit board are attached, a solder paste stencil is applied to the circuit board. The solder paste stencil is a thin metal stencil that has areas cut out that correspond to the areas of the circuit board where the components will be attached. Solder is then applied. The solder goes through the areas cut out on the stencil such that solder is deposited on areas of the circuit board where the leads of the components will be attached. The leads of the components are placed on the solder, soldering the components onto the circuit board. Screws are then added to hold the back plate and circuit board together more securely.
A problem with fabricating the assemblies is that it is difficult and expensive to make an epoxy that can bond the metal back plate to the material of the circuit board. The epoxy must be tailored to the circuit board and back plate. The development of the epoxy takes both time and money, therefore increasing the cost and the time to start production. However, even when the epoxy is tailored to the circuit board and the back plate, the epoxy is not very reliable. A significant percentage of the circuit boards detach from back plates. This reduces the yield and therefore raises the cost and time of fabricating the assemblies.
Attaching the components to the circuit board prior to attaching the circuit board to the back plate solves the problem of using the epoxy, however, it creates a more serious problem. FIG. 1 shows circuit board 4 with components 6, 8 attached to circuit board 4 before circuit board 4 is attached to back plate 10. Attaching components 6, 8 to thin circuit board 4 before circuit board 4 is reinforced with back plate 10 causes circuit board 4 to warp and bend due to the mismatch in the thermal expansion coefficient between the board and the components 6, 8. This warped circuit board 4 is then screwed onto back plate 10. Back plate 10 is more rigid than circuit board 4 causing warped circuit board 4 to bend when it is screwed on back plate 10. This can cause the printed wiring on circuit board 4, circuit board 4 itself, components 6, 8, or the solder joints on components 6, 8, to crack, which disrupts connections and causes open circuits. This is particularly problematic when the crack in the printed wiring is difficult to detect, and the assembly is placed in a product and fails in the field.
Attaching the back plate to the circuit board with screws before soldering eliminates the need for the epoxy but it creates a more serious problem. FIG. 2 shows assembly 12 with screws 14, 16 attaching circuit board 18 to back plate 20. Back plate 20 also has openings 30 and 32. Openings 30 and 32 can be used for fasteners that fasten assembly 12 to the devise that receives assembly 12. Circuit board 18 also has openings 26 and 28. Openings 26 and 28 can be used for fasteners that fasten assembly 12 to the device that receives assembly 12. Back plate 20 and circuit board 18 include these openings 30, 32, and 26, 28, respectively, in addition to the opening for the screws 14, 16 that attach circuit board 18 to back plate 20. Circuit board 18 is typically too soft to be able to have threads to hold screws 14, 16 in place if their heads 22, 24 are positioned at the bottom of back plate 20. Therefore, heads 22, 24 need be positioned above circuit board 18. Heads 22, 24 have to be above at least a portion of circuit board 18 otherwise they will not hold circuit board 18 to back plate 20. In the cases of all but the thickest circuit boards, heads 22, 24 are taller than, or at least as tall as, the thickness of circuit board 18. Since heads 22, 24 need to be positioned above at least a portion of circuit board 18, at least a portion of heads 22, 24 protrudes above top surface 34 of circuit board 18.
Heads 22, 24 protruding over the top surface of circuit board 18 create raised areas in the top surface of assembly 12. If the solder paste stencil is placed on top surface 34 of circuit board 18, a gap is created between circuit board 18 and the stencil. The solder is then applied and will leak out onto circuit board 18. This will cause the solder to be deposited on areas of circuit board 18 where it does not belong, i.e. the areas of circuit board 18 where components do not need to be attached, resulting in short circuits. When components 36, 38 are attached to circuit board 18, some components 36 may attach to the solder that is deposited where it does not belong, resulting in incorrect connections and to open and short circuits. Depositing solder manually onto the circuit board instead of using the solder paste stencil, to avoid the solder leaking over the assembly, increases the cost and the time needed to manufacture assembly 12 to the point where it is prohibitively expensive to manufacture assembly 12.